A conventional flow measuring instrument employs the sing-around method that improves the measuring resolution by repeating a transmission and reception between a pair of oscillators for multiple times. FIG. 9 shows a conventional flow measuring instrument for fluid, which employs the sing-around method. The instrument is formed with a first oscillator 52 for transmitting an ultrasonic wave, a second oscillator 53 for receiving the ultrasonic wave transmitted, both oscillators 52 and 53 being disposed on a fluid conduit 51, and a measurement control section 54 for measuring a travel time of the ultrasonic wave traveling between the oscillators 52 and 53.
Assume that the velocity of the ultrasonic wave is “C”; a flow velocity is “v”; a distance between the pair of oscillators is “L”, and an angle of the traveling direction of the ultrasonic wave with respect to the flow direction is θ. Assume also that a travel time of the ultrasonic wave transmitted from the oscillator placed upstream of the conduit to the other oscillator placed downstream thereof is “ta”; and a travel time in the opposite direction is “tb”. Then, the travel times “ta” and “tb” can be expressed by the following equations:ta=L/(C+v×cos θ), and tb=L/(C−v×cos θ).
The velocity “v” can be derived from the above two equations as follows;v=L×(1/ta−1/tb)/2 cos θ.
The velocity “v” thus derived is multiplied by a sectional area “S” of the conduit and a correction coefficient “K”, so that an instant flow volume “Q” of the fluid can be expressed as Q=v×S×K.
When the velocity “v” is small, a difference between “ta” and “tb” is also small, so that it is difficult to measure the difference accurately. It is thus necessary to repeat the measurement for multiple times and average the results to minimize associated errors and improve the measurement resolution.
This measurement operation is implemented as follows: An ultrasonic wave is repeatedly transmitted for “n” times from the upstream side to the downstream side. Assume that the total time necessary for the “n” time-transmission/reception is “Ta”. Then, the ultrasonic wave is likewise repeatedly transmitted in the opposite direction for “n” times. The total time necessary for this opposite “n” time-transmission/reception is “Tb”. “Ta” and “Tb” thus obtained are divided by “n” respectively to derive respective travel times “ta” and “tb” per one transmission. The travel times “ta” and “tb” thus averaged are used in the equation for deriving an instant flow volume “Q”, whereby an accurate flow volume can be calculated.
Such an measurement operation is carried out repeatedly at regular intervals “τ” e.g. every two seconds, and each of the derived instant flow volumes “Q” is multiplied by this time interval “τ”, whereby volumes of fluid which passed through the conduit 1 for the time intervals “τ” can be obtained. The results are integrated to thereby obtain a total volume of the fluid flow.
However, the measurement operation discussed above has a problem that under conditions that a fluid flow cyclically changes in such a manner that the flow velocity changes during the time interval “τ”, the measurement results necessarily contain errors depending on phases of the cyclically changing fluid flow in which the measurement is carried out.
To overcome this problem, unexamined Japanese Patent Publication No. 2003-28685 discloses a method in which an average of flow velocities is calculated in each of the phases of the cyclically changing fluid flow to thereby obtain an accurate flow volume. FIG. 10 shows a timing chart illustrating this method, which shows timings of measurement along flow changes. Starting at time “τa1”, travel times of ultrasonic wave are measured at four times which propagates between the first oscillator 52 (transmitter) placed upstream and the second oscillator 53 (receiver) placed downstream. Then, the measurement control section 54 switches the role of the first oscillator 52 and the second oscillator 53, and starting at time “τb1”, travel times are similarly measured at four times.
These travel times measured in both directions are referred to as a first set of measurements, and given sets of measurements are then performed. Further in FIG. 10, starting at time “τa2”, measurements are performed with the first oscillator 52 (transmitter) and the second oscillator 53 (receiver), and starting at time “τb2”, measurements are again performed with the first oscillator 52 (receiver) and the second oscillator (transmitter). These measurements are referred to as a second set of measurements.
The travel times in both directions are measured for “m” sets (“m” is an integer). The measurement control section 4 then obtains the total travel times “Ta” for the measurements performed, starting at time “τam”, with the upstream transmitter, and the total travel times “Tb” for measurements performed, staring at time “τbm”, with the downstream transmitter. “Ta” and “Tb” are divided by the number of measurements (4×m) to obtain averages “ta” and “tb” per a measurement, using which an average flow volume of the fluid is thereafter calculated. This series of operations is carried out at regular intervals, whereby the integrated flow volume can be obtained.
By choosing a proper time interval between the respective sets of measurements and a proper number of measurement sets, measurements can be performed in all the phases of the cyclically changing fluid flow, so that an accurate flow volume is obtainable.
However, in the above-explained conventional method, during a series of measurements which consists of a given number of sets of measurements, samplings are carried out at very short intervals, e.g., every several milliseconds, and results of respective samplings are integrated. It is also necessary to keep supplying power to the electronic circuits forming the measurement control section in order to control the intervals between the measurements, control the number of measurement sets, and store the integrated results. The electronic circuits thus consume a lot of electricity, and if used in a gas meter installed outside a house, need a battery of a large capacity.